| With the abbreviation of PLC, Programmable logic controller plays an extremely important role in the industrial area. Nowadays, the increasing application of industrial control asks for PLC with higher performance, especially the reliablity. Generally, the reliability of PLC can be improved from hardware and software aspect. On the hardware side, it enhances the fault-tolerant capability of system and ensures that system works normally when failure happens by redundancy. However, it will leads to increased development costs, and also increases the complexity of the development process. In terms of software, it depends mainly on the self-detection of PLC or the use of diagnostic techniques. If the function of fault diagnosis is added to system by the traditional PLC, it will expand consequentially the scale of application program and extend the scan cycle of PLC, resulting in the difficulty to meet timely response of real-time controlling. How to improve system reliability without affecting the timely response to the control task is the major issue studied in this paper.Embedded PLC, which is a perfect combination of embedded system and PLC, realizes the interpretation and implementation of PLC control software code in embedded system. This paper uses embedded PLC to achieve high reliability, as the embedded PLC has high cost performance and allows multitask, which makes up for the shortcomings of the methods used on software and hardware by traditional PLC. The main work of this paper can be divided into two parts.Firstly, embedded PLC with ability of soft fault diagnosis is achieved in this paper. Data acquisition task, a subtask of the fault diagnosis system, which sends diagnostic data to the diagnostic system, is the only part embedded into the PLC control system in order to prevent affecting the original control performance. Coordinate the synchronous operation of control task and data acquisition task by preemptive priority scheduling algorithm and the tasks of data acquisition, realizing that control task runs cyclely and data acquisition task runs during the spare time of control cycle. Then the design proposal of the associated memory management scheme and process communication strategy is studied. What’s more, we discuss the different Communication demands of the two types of data in embedded PLC, giving the strategy of dual serial communication.Secondly, we use the technology of state machine to establish a fault diagnosis system on PC. Initialize the specification with diagnostic data of embedded control system in normal operation. Then gather data from the actual control system, combined with passive testing algorithm for error detection, which can help us to detect unknown status or diagnose soft fault. When fault is found, corresponding prompt must be displayed to users. If unknown status is detected, it won’t be confirmed until the maintenance staffs determine whether the fault condition exists, which will also improve the standard model. To reduce memory consumption of the diagnosis system, the technology of route coding is chosen to compress the state machine.This paper provides a heuristic method to explore and research PLC with high reliability. This method is conducted on the elevator control system experimental platform, and the operation of opening and closing the door is chosen to train specification and diagnose. The experimental results verify the feasibility and practicality of this method. |
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